Etymologies

(American Heritage® Dictionary of the English Language, Fourth Edition)

From Middle English spang ("a small piece of ornamental metal; spangle; small ornament; a bowl or cup"), likely from Middle Dutch spange ("buckle, clasp") or Old English spang ("buckle, clasp") (Wiktionary)

Set roasted beef and pudding on the opposite side o 'the pit o' Tophet, and an Englishman will mak a spang at it -- But I wash my hands o't -- Follow me sir "(to Andrew)," and I'se show ye where to pit the beasts. "